Field of the Invention
This disclosure relates to an injection molding resin gear and a method of manufacturing the injection molding resin gear and, more specifically, to an injection molding resin gear including a tooth portion having teeth formed on an outer periphery thereof, a shaft supporting portion formed around a center of rotation, and a web that connects the tooth portion and the shaft supporting portion.
Description of the Related Art
In the related art, a number of injection molding resin gears are used as a power transmitting component for machines such as copying machines and printers. In this field of application, requirement of accuracy of the injection molding resin gears is significantly increased for improving the quality of printing.
The injection molding resin gear is molded by infusing resin from multi-point gates opening on a web surface. In this case, since the resin infused from the gates is filled in a cavity radially, the resin reaches the tooth portion on an outer peripheral portion earlier than an intermediate position between the gates in the circumferential direction of a gear at positions matching the gates in the circumferential direction of the gear, so that resin filling to the tooth portion becomes uneven, which adversely affects a tooth plane accuracy standardized by an engagement error (Japanese Gear Manufacturers Association standard number 116-02 (JGMA 116-02)) or the like.
As a method of enhancing the accuracy of the injection molding resin gear, a method of disposing an annular groove shape for rectification at a position in the midcourse of a resin flow to rectify the flow is known. As described in Japanese Patent No. 4173053, a method of disposing a groove shape in the vicinity of the gates to restrain a resin flow (known technology) is reported. Alternatively, as described in Japanese Patent Laid Open No. 2011-31470, a method of forming a rib so as to project upright on a portion on the outer peripheral side of the gates of the web, adjusting the height of the rib so as to be high at positions corresponding to the gates and low in the intermediate positions between the gates in the circumferential direction to form buffering areas which diverse the resin flow from the gates toward the outer peripheral portion, whereby complementing resin reaching timing to the outer peripheral portion of a mold is reported.
However, with the annular groove shape for rectification provided in the midcourse of the flow of the related art or with the method of disposing the groove shape in the vicinity of the gates as described in Japanese Patent No. 4173053, in order to obtain an advantage of sufficient improvement of tooth plane accuracy, if the web is a gear having a large diameter and a large thickness, no problem arises. However, since achievement of high-accuracy and compact configuration is in progress, in the case of high accuracy injection molding resin gear having an outer diameter as small as Φ30 mm to Φ45 mm and a thickness of 1.5 to 2.0 mm such as color printers and copying machines of the recent years, if the groove for achieving the effect of rectification is disposed, there arises a problem of lowering of the strength of the gear due to the thinning of the groove portion, and lowering of accuracy of a tooth plane and the shaft supporting portion because the groove shape is provided as a position close thereto.
In Japanese Patent Laid Open No. 2011-31470, since the rib is disposed on the outer peripheral side of the gate, an influence on the accuracy of the tooth plane is a concern in the case of the injection molding resin gear having a small diameter. In addition, in order to obtain a sufficient effect, the thickness of the rib needs to be increased, and hence deformation of the web is a concern. Alternatively, the height of the web needs to be changed right below the rib, so that the flexibility of a design may be reduced.
In this manner, in the related art, there is a problem of unevenness due to resin flow caused by the arrangement of the gates in the web.